Do you mean a USB flash or USB HDD? If the latter, then there could be only workarounds as Acronis Snap Deploy can only place a bootable component onto a USB flash drive. The easiest workaround was suggested by one of our experienced MVPs here https://forum.acronis.com/comment/350328#comment-350328

"A work-a-round is to create a bootable flash drive with another "removable" type. then Image it with Acronis... then restore that image to your "fixed" usb drive. It now works. You can also use this method to add Acronis bootable media to a larger external hard drive. However, be advised that restoring an image to a single parition drive will format it completley. Instead, you may want ot parition about 1GB free at the begiinning of such a disk and restore the image to it so that the remainng data is not impacted."